Machines, particularly machines with rotating parts, such as gas turbine engines, typically undergo cyclic loading during operation. Over time, thermal and mechanical stresses resulting from cyclic loading may cause machine components to fatigue and develop cracks. The rate at which cracks develop and propagate has a direct impact on the lifetime of machine components. By monitoring the growth of cracks on test specimens of machine components, it may be possible to better estimate the useful lifetime of machine components.
One method of monitoring the rate of crack growth measures a change in electric potential across a pre-established crack in a test specimen carrying an electric current, such as provided in ASTM Method E647. The test specimen is then subjected to high temperatures and stresses that replicate the environment inside a working machine. As the crack propagates under the intense thermal environment and stresses, the voltage across the crack increases. This change in electric potential can be measured using two sensing leads placed on either side of the crack.
Instrumentation for monitoring crack growth 100 is shown in FIG. 1, a cross-sectional view. The instrumentation 100 includes a test specimen 110. A starter crack 120 is created in the test specimen 110. Current leads 140, 141 are welded to the test specimen 110. Sensing leads 130, 131 are attached to the test specimen by tack welds 125, 126. The current leads 140, 141 are attached to a standard direct current (DC) or alternating current (AC) source (not shown), while the sensing leads 130, 131 are attached to a voltmeter or other similar device (not shown) to measure the voltage change across the starter crack 120. The test specimen is exposed to conditions which attempt to replicate service conditions, which include high temperatures and stresses. Any growth in the starter crack 120 caused by these conditions causes a change in potential across the starter crack 120 which can be detected using the sensing leads 130, 131.
One problem associated with conventional instruments, such as the one shown in FIG. 1, is that even if the sensing leads are attached to the surface of the specimen with a only a low power weld, some small cracks or weak spots result in the specimen at the welds which invalidates test results. The ability to measure crack propagation is limited to starter cracks larger than 4 mils deep and 8 mils wide, as interference develops from the weld cracks or defects when the starter crack is smaller than this size, making it difficult or impossible to distinguish between propagation of the starter crack and propagation of the defects associated with the tack welds. Further, the sensitivity with which crack growth can be determined depends upon the distance from the sensing lead to the starter crack. The closer the sensing leads are attached to the starter crack, the more likely that any cracks that form at the welds will be a source of interference in measuring electric potential change and thus growth of the starter crack. Thus, conventional methods of using a change in electric potential to measure crack growth propagation are also limited in the sensitivity at which increments of crack growth can be measured. This physical limitation is undesirable.
Accordingly, it may be desirable to provide instrumentation and methods to monitor the growth of cracks that limit inherent damage to the specimen, which damage interferes with the sensitivity of the instrumentation.
It may also be desirable to provide instrumentation and methods that are more sensitive, with the ability to monitor the growth of particularly small cracks, such as those smaller than about 4 mils deep or 8 mils wide, which may result in the ability to even better predict component lifetime.